The claim goes that KFC had to change their name from Kentucky Fried Chicken to KFC because they had started using chickens that are so genetically modified as to be no longer classifiable as 'chicken'. The specifics of this rumour vary but generally the description involves featherless and beakless chickens, pumped with steroids to provide maximum meat through minimum work.
Here's an example of the viral text that has most recently been doing the rounds: